40 MOSTLY MAMMALS 
ment; while the goat can obtain a living on ground where 
a sheep would starve. Moreover, the ass and the mule 
replace the horse in arid and mountainous countries, where 
they thrive on a much less luxuriant diet than is necessary 
to the well-being of the latter animal. 
Then there are climates in which many of the domesti- 
cated animals of Europe will not flourish, dying either 
from the general effects of the climate itself, or succumbing 
to the attacks of insect-pests, as in the familiar instance 
of the African tsetse-fly. 
As regards the supplementing of the existing domesti- 
cated animals of Europe—whether they be used for labour 
or for food—by newly domesticated wild species, I venture 
to think that, in the main, there is very little chance of 
success. In the first place, the species we now possess in 
this condition are amply sufficient to serve all needs, and 
are capable of indefinite multiplication. And in the second 
place, it has to be borne in mind that it would probably 
take scores of generations to make a wild animal equal in 
point of utility to the old-established domestic breeds— 
that is to sdy, it would take an immensely long period 
of time in order to make any wild animal as immune to 
the effects of in-and-in breeding as is the case with our 
domesticated species; while it is quite likely that the time 
would be still longer before the former would approach 
many of the latter in flesh-forming power or in the capacity 
for early maturity. And in this connection it is most 
important to bear in mind that the great majority of our 
domesticated animals are very different in physical characters 
from their wild ancestors; and that, in most instances, it 
is these highly modified breeds that are of the greatest 
economic importance to mankind. To produce an animal 
like the sheep, for instance, which differs from all its wild 
